The present invention relates to a device for the passing of the stitch from a needle for the forming of plain knitting to a needle for the forming of purl knitting, and vice versa, in a knitting machine and the like.
In the field of knitting machines and the like devices are known for the transfer of one or more stitches, or loops, from needles which form plain knitting to needles which form purl knitting in order to obtain particular knitting patterns.
In particular in knitting machines with a cylinder and platen, said devices are essentially composed of a shaped small plate associated with the side of a first needle for the forming of plain knitting, respectively purl. The small plate has its end opposite to the point of the needle rigidly associated with the side of the needle, whereas the other end is accommodated in a seat defined on the side of the needle. Said small plate is elastically flexible and defines together with the side of the needle a loop in which a second needle may be inserted for the forming of purl knitting, respectively plain, to hook onto the last loop formed by the first needle which is retained by a shoulder provided on the first needle, around the loop.
In practice the advancement of the first needle causes the sliding of the loop along its stem, the tab of the needle is opened by the loop which is retained by the shoulder of the needle on the loop in which the second needle penetrates. The subsequent return of the first needle causes the hooking of the loop by the second needle and the exit thereof from the loop with an elastic flexing of the small plate.
In knitting machines with not high refinement values, instead of using a small plate associated with the side of the needle, a needle is used with a hollow on a side in which can be inserted the needle which must receive the loop.
With these kind of devices the passage is obtained of the last loop formed by a needle of the cylinder to a needle of the platen and vice versa, thus obtaining a change in stitch from plain knitting to purl knitting or vice versa.
Said known types of device, though they obtain the transfer of the stitch from the plain knitting to the purl knitting, are not however free from disadvantages.
In particular in case of high refinements, a difficulty is encountered in the passing of the stitch since the small plate must be extremely flexible in order to be contained within the grooves of the cylinder, or of the platen, with the needle to which it is fixed and this great flexibility does not allow for a widening of the loop sufficient to allow for the insertion of the needle which must receive the loop, with consequent breakage or loss of the yarn.
This fact is all the more evident the more the yarn is stiff, such as, for example, during the knitting of cotton yarns.
Furthermore, since the small plate, when accommodated within the groove, is compressed between the side of the needle and the side of the groove, there are unavoidable problems of wear even during normal knitting, which affect the lifespan of the small plate.
Problems of wear are also found on the point of the needle which receives the loop for the friction which occurs, when the stitch is hooked and slipped off the needle which casts away the loop, between the needle which receives the loop and the point of the small plate which is opened by the needle.
In the case of not high refinement, there are problems of wear for the interference of the needle which casts away the loop with the needle which receives it; said interference causes the elastic deformation of the needles.
Another disadvantage is due to the fact that, in order to perform the transfer of the loop, it is necessary to make the needle which casts away the loop come out of the groove of the platen, or of the cylinder, of an amount far greater than that required by normal knitting. This fact implies that, in a circular knitting machine a special sector is provided for performing this operation, in which sector yarn-guides are absent in order to avoid interference with the points of the needles. Furthermore, the small plate with the loop determines a width of the groove which causes an excessive slack of the needle, which is fully disadvantageous for the precision of the knitting.
From U.S. Pat. No. 2,040,319 is known a device constituted by a small plate which flanks longitudinally a first needle, which works plain or purl, and accommodated with the same in a same groove of the cylinder of the needles or of the platen. The side of the needle facing the small plate has a shaped profile, which can be associated with a tooth defined on a portion of the small plate in such a way that a longitudinal sliding of the small plate relatively to the needle causes the moving away of a section of the end of the small plate orientated towards the point of the needle in order to define a loop in which a second needle can be inserted, which needle knits purl, or plain, so as to take the last loop formed by the first needle which is appropriately retained around the loop.
Such a kind of device, though it overcomes the disadvantages denoted by the previously described types, presents in turn some disadvantages.
One of these is due to the fact that the tooth of the small plate is in contact with the shaped profile of the needle with a reduced surface, and therefore relatively high stresses occur, which bring about a rapid wear of the point of the tooth, in particular in machines with high operating speeds. Furthermore, due to the fact that the tooth is defined on a portion of the edge of the small plate, there may occur, during the relative sliding of small plate and needle, an accidental slippage in a transverse direction of the small plate, with a consequent disengagement of the tooth from the shaped profile of the needle, in particular in the case of scarcely elastic yarns.